Bearings used in high speed spindle applications, including electric motors, electric generators, and electric machines, are typically comprised of an inner ring and an outer ring, which each have raceways on which rolling elements roll, and cage assemblies.
The passage of electrical currents through bearings is undesirable and can cause damage to the bearing raceways and rolling elements. In some circumstances, the damage caused to bearings can ultimately lead to bearing failure. Damage caused by electric current passing through bearings commonly leads to “fluting.” Fluting refers to washboard wear patterns that develop along bearing raceway surfaces in the direction of rotation as a result of a combination of the continuous flow of electric currents and vibrational properties of bearing components.
In addition to damage or failure of bearings, electrical current can affect the machines associated with the bearings. The damage caused by the passage of electric currents can be severe, causing high noise, premature failure, and shutdown of a machine. Also, other machines can be affected negatively through the same stray electrical currents or even small amounts of static electricity. Significant costs and lost revenue can occur as a result of the failure of a machine. In addition to the repair costs associated with the failure of a machine, the costs are compounded by the revenue loss of the machine downtime during repair and resulting production loss.
The electrical currents which are induced onto bearings by machines are difficult to eliminate. However, damage to bearings can be prevented or at least significantly reduced by taking preventative measures. Known preventative measures include current insulating or current conducting ceramic ball bearings, ceramic coated outer bearing rings, electrical conductive grease, or a shaft seal, which is not integrated in a bearing, but rather into a shaft seal unit, with integrated current grounding features.
Bearings using ceramic rolling elements for insulation from electric currents are known. Ceramic has a very high resistance to passage of electric current, and thus these bearings typically exhibit a higher resistance to the passage of electric currents than standard bearings. However, ceramic rolling elements do not ensure electric currents will not lead to bearing failure. Electric currents can jump given enough potential and cause damage to the bearings. Moreover, ceramic rolling elements are expensive.
Alternatively, see, for example. U.S. Pat. No. 6,508,591, which discloses conductive ceramic rolling elements made of conductive material. There, conduction is limited to only the rolling element, which is undesirable. Also, similar to the insulating ceramic rolling elements, the process of manufacturing a rolling element of conducting material is expensive.
Moreover, bearings utilizing ceramic coated outer rings for current insulation are known. However, the process of applying an insulating ceramic coating to the outer ring is expensive and the layer of coating can wear down. Also, insulation is limited to only the outer ring, not the entire bearing.
Bearings utilizing electrical conductive grease are also known. However, grease has undesirable properties, when used as a lubricant, primarily carbon particles, that interfere with the bearing function and act like contaminants. Moreover, electrical conductive grease does not provide satisfactory current conduction of electrical currents in most cases.
Additionally, see, for example, U.S. Pat. No. 7,498,707, which discloses a shaft seal with integrated current grounding elements or brushes that form an electrically conductive path between a shaft and an electrical ground to thereby dissipate residual current and/or stray shaft voltage. Here, the seal is installed, independent of the bearings on the shaft, between a housing and a stator. The seal is not integrated into the bearings. Thus, additional space is required to include the seal in the application and the seal does not prevent the transmission of electrical current via a bearing. See also, U.S. Patent Publication No. 2006/0007609, which relates to a shaft current control brush ring assembly for another example of a shaft seal.
Moreover, other shaft seals are known, which utilize carbon brushes that have electrically conductive carbon graphite for shaft and current grounding. A graphite film typically is deposited on a contact area of carbon brushes during sliding. However, it is commonly known carbon brushes experience excessive wear and hot-spotting/thermal molding and brush dusting/low humidity lubrication.
Furthermore, bearings utilizing slip rings, dissipative brushes, and non-conducting bearing closure elements, such as shields or seals, are known. However, incorporating dissipative brushes into a bearing requires the width of the bearing to be increased. Also, wear from slip rings is undesirable and must be absolutely minimized or kept away from the bearing. Finally, existing bearing closure elements do not contain current insulating or conducting features.